A polarization stabilizer is a device that transforms an input optical radiation having an arbitrary input state of polarization (SOP) into an output optical radiation with a predetermined SOP (typically but not necessarily fixed) and with an optical power, both not dependent on the input SOP (i.e. the ratio between output and input optical power is independent of the input SOP). In general, a defined SOP is determined by two parameters: the ellipticity and the polarization azimuth.
In optical communication systems, such a device is useful, for example, placed immediately before (upstream) a polarization sensitive optical component for compensating the random SOP fluctuations of an optical radiation after a link made of, e.g., single-mode optical fibers. Examples of SOP sensitive optical components are integrated-optics components, coherent optical receivers (wherein a match is sought between the SOP of the signal and the local oscillator), fiber optic interferometric sensors, polarization mode dispersion compensators (wherein an alignment is sought of the two orthogonal output principal states of polarization with respect to a variable delay line introducing a delay between two fixed SOP) and the like.
In polarization division multiplexing (PolDM) transmission systems, at least two channels are launched orthogonally polarized in the optical transmission medium, such as for example an optical transmission fiber. In a typical solution for PolDM transmission, the at least two channels orthogonally polarized are closely spaced, such as for example within 50 GHz spacing or within 25 GHz spacing. In a preferred configuration, the two channels have substantially the same optical wavelength. Typically, while the reciprocal orthogonality of polarization is substantially preserved along the propagation into the transmitting medium, the SOPs of the two channels randomly fluctuate in time at a given position along the line, such as for example at the receiver section.
In PolDM, whenever the two orthogonally polarized channels have to be polarization demultiplexed, typically a polarization beam splitter is used as a demultiplexer, which is apt to split two orthogonal SOPs. In such an application, exact polarization stabilization of the SOPs of the two channels is strongly desired, in order to facilitate polarization demultiplexing. In case of an error in polarization locking, a misalignment occurs between the SOPs of the two channels and those of the demultiplexer. In this case a cross-talk is generated due to an interference between a channel and the small portion of the other non-extinguished channel, which severely degrades the quality of the received signal.
A fundamental requirement of a polarization stabilizer is the endlessness in control, meaning that the stabilizer must compensate in a continuous way for the variations of input SOP.
Several polarization control schemes based on finite range components have been presented. In such schemes, in order to achieve an endless control, it is necessary to provide a reset procedure when a component reaches its range limit so that the output SOP does not change during the reset. Generally, reset procedures can be problematic in that they are often associated with complex control algorithms designed to avoid loss of feedback control during the reset.
WO03/014811 patent application, which is herein incorporated by reference, discloses an endless polarization stabilizer based on a two-stage configuration wherein the two stages are controlled independently by an endless polarization stabilizing method based on a feedback control algorithm. Each stage comprises a pair of birefringent components that each have fixed eigenaxes and variable phase retardation. The two birefringent components are variable retarders with finite birefringence range and respective eigenaxes oriented at approximately ±45 degrees relative to each other. The endlessness is obtained by commuting the phase retardation of one retarder, when the retardation of the other retarder reaches a range limit.
WO06/027205 patent application, which is herein incorporated by reference, discloses an endless polarization stabilizer based on a two-stage configuration wherein the two stages are controlled independently by an endless polarization stabilizing method based on a feedback control algorithm. Each stage comprises a pair of variable rotators, each having fixed eigenstates and variable phase retardation, and an optically interposed quarter-wave plate, and a controller configured for switching between first and second values the phase retardation of the first of the variable rotators, whenever the phase retardation of the second of the variable rotators reaches an upper or a lower limit. The upper and lower limits of the second rotator and the first and second values of the first rotator are chosen so that discontinuities in the power of the output optical radiation are avoided when the first variable rotator is switched, thereby providing endless polarization stabilization using rotators that themselves may have limited retardation ranges.
The article “Polarization control in advanced optical communications systems” by M. Martinelli et al., proceedings of 2006 China-Italy Bilateral Workshop on Photonics for Communications and Sensing (Xi'an, China, 23-24 Oct. 2006) discloses theoretical and experimental aspects of polarization stabilization. WO2006/045324 patent application, which is herein incorporated by reference, discloses methods and devices for stabilizing the state of polarization of a polarization multiplexed optical radiation, as well as related polarization division multiplexing optical communication systems.